Cold engine problem

[Zigomatix]

Hi there,
Here is what happens every time I start my NX after a long stop (e.g; all night or all day off) :
I turn on fuel, lights off.
I put choke in full position.
I push the start button, several seconds are needed but I don't need to open throttle.
Engine is working for a few seconds at 2-3k rpm then jumps to 6-7k rpm so I have to turn choke down (but not off) carrefully because if I am too fast, engine stops.
I have to check down choke regulary while I put helmet and gloves on because engine increases rpm by steps.

Finaly I start riding, engine is not totaly hot, I keep a little choke for a few km (1 or 2) then I turn it off.
While riding, and engine temperature not stabilisated, if I go full throttle fast, no explosions. I realease throttle, explosions are back.

When engine is hot, after even 1/2 hour stop, I can start it with no choke. A 1s push on the start button is enough. I can also turn full gas fast and it works. Also idle rpm are stable around 1300, 1400 rpm.

Any idea of what is wrong ?
Last but not least I can't get under 3.3 l/100 km ...
I have to mention that I have 130/90 rear tyre and 1 tooth less front sproket .

Patrick

[HRC]

Does not sound unusual to me.

[Garfield]

... if I go full throttle fast, no explosions. I realease throttle, explosions are back.

Patrick,

What do you mean with this exactly ?
Do you mean that the engine stalls if you pull the throttle very fast ?

Could be that your accelerator pump does not work properly or that your idle mixture is not correct, maybe due to a clogged pilot jet.

For the rest it looks quit enormal what you describe, or I must be misunderstanding something.

A cold engine should be started with choke on but no throttle.

[Zigomatix]

Do you mean that the engine stalls if you pull the throttle very fast ?

Yes that is what I tried to explain with my poor english
This only happens when engine is not at his right temperature.

I will try to clean the pilot jet. Is it possible without disassembling all the carburetor ? I'm affraid not to be able to set it back correctly ...

What is the accelerator pump ?

For the rest it looks quite normal what you describe, or I must be misunderstanding something.

A cold engine should be started with choke on but no throttle.

Ok I try to be more precise :
Engine cold start -> 2 or 3 seconds @ 2-3k rpm then jumps to 7 - 8 k rpm (choke 100%)
Choke adjusted (approx 40 - 50 %) to set engine at 2 - 3 k rpm -> 10 to 20 seconds later jumps to 5- 6 k rpm
Again choke adjust (approx 10 %) to set 2 k rpm
There I usualy go riding.
What bother me is that rpms are jumping, not increasing regulary.
If you say it is a normal, ok for me too, I was not sure ...
It's been a long time I had a carburetor bike with manual choke and it was a 2 stroke ... All my orther bike either had automatic choke or EFI and they don't do that...

Thanks for your replys
Patrick

[Garfield]

Yes I consider the jumping of the rpm rather normal. I have and have owned several bikes (VFR, Pan-European, Transalp) that all more or less do the same.
I would not worry about that.

You can access the jet by unscrewing the float bowl underneath the carburetor.
However, you would need to take the carb off to do that and that will take quite some more disassembling.

You could try a cleaning agent first, from Wynn or somehting like that.

The accelerator pump is a nifty contraption that will squirt extra fuel in the inlet when you open the throttle quickly.

Has your bike been standing still for quite some time ?
Has it been fed E10 fuel ?
That could cause clogging in the internals of the carb.
Never use E10 in these old bikes, go for E5 always.

[HRC]

In France E10 is more common than E5. According to Honda it is no problem for 1989 and newer modells of the NX250.

For engines with carbourators it is not unusual to stall when openig the throttle rapidly at low RPMs or at low engine temperature.

[Garfield]

Here in NL E10 is also more common and yes I am aware of what manufacturers states but I have seen too many problems as a result of E10 already.

I missed that the stalling occurs when the engine is not fully warm yet.
If the engine is warm, pickup should be very fast and smooth when you open the throttle, that is what the accelerator pump is for.

[scrambler]

Engine cold start -> 2 or 3 seconds @ 2-3k rpm then jumps to 7 - 8 k rpm (choke 100%)

that is waaaay too much. That happens often because of the combined AC pump /cold start adjuster. So you start with ajusting the ac pump, the gap should be around 0,5mm. If you would like less fuel consumption you could choose a wider gab. After that you can adjust the cold start rev by turning the screw - according to the manual it should around 3000rpm with warmed up engine.

[Zigomatix]

hi, thanks to all for answering.
It was, in, fact, my mistake : When I change my choke cable, I adjusted cold start adjuster as there was no gap.
Yesterday, it was time for 30 000 km service so I set this gap to average position and see how it goes. I also changed engine oil (and filter) and cleaned and checked the spark plug as well as the air filter.
Now bikes start from all night stop normaly, I don't have to pull the choke 100% on anymore and rev jumps to 3 k rpm almost immediately so I can adjust it and let the bike heat while dressing up.

What do you think about my spark plug color ? sorry for the picture, I have no better image.

spark plug.jpg

I run E5, never used E10 in this bike. In france E5 is 98% octane as E10 is 95% (both are unleaded of course).
I heard about the 10% alcohol from E10 was not good at all for rubber seals (specialy old ones).

Patrick

[scrambler]

looks good to me - the NX is pretty much on the lean side, so the isolator is usually white. I use E10 from time to time with no negative effects - as far as I know it's only bad when you leave it for more than 4 weeks in your carb without driving.